Question

The proof, with a missing reason, proves that the measure of angle ECB is 54°.

Answer 1

Answer:

Midsegment of a Triangle Theorem

Step-by-step explanation:

$In\: \triangle ADE\\m\angle A + m\angle D + m\angle AED = 180°\\\therefore 90°+36°+m\angle AED= 180°\\\therefore 126° +m\angle AED = 180°\\\therefore m\angle AED = 180°-126°\\\therefore m\angle AED = 54°$

$In\: \triangle ADE,$ D and E are mid points of sides AB and AC respectively.

Therefore, DE || BC

Hence, by Midsegment of a Triangle Theorem

$m\angle ACB = m\angle AED =54°(corresponding\:\angle 's) \\\therefore m\angle ECB = m\angle AED =54°(\because A - E - C)$